A total of 91 boys underwent bilateral orchiopexy with testicular biopsy. The percentage of tubular transverse sections containing spermatogonia, the fertility index (FI) and the number of spermatogonia per tubular transverse section (S/T) were measured and expressed as a percentage of the minimum normal value (FI% and S/T%, respectively). The maximum and mean of the 2 FI% and S/T% values were noted for each patient. At follow-up, testicular volume was measured in 73 men; 71 of these gave a blood sample for FSH and 67 provided specimens of semen for analysis. The maximum and the mean FI% and S/T% values of the patients were positively correlated to maximal sperm density and to volume of pair of testes and negatively correlated to serum FSH. It seems reasonable to use these histological parameters as prognostic indices of fertility potential. In adulthood only 11 patients had sperm density greater than or equal to 20 million/ml, 37 had normal serum FSH and 29 had bilateral testicular volume greater than or equal to 15 ml. Patients with both testes originally placed in the superficial inguinal pouch at surgical exploration had a higher maximal sperm density than the rest of the study group.